Avoid adding duplicates to PATH

Without this, each time the env script gets executed it will add a copy
of the paths to PATH. This can happen when shells are nested.

The "case" trick was inspired by:
https://github.com/rust-lang/rustup/blob/184a899f4529f27e23eca537a5979c530ff7fa26/src/cli/self_update/env.sh

scripts/bootstrap/bootstrap-haskell

@@ -280,7 +280,20 @@ download_ghcup() {
 
 	# we may overwrite this in adjust_bashrc
 	cat <<-EOF > "${GHCUP_DIR}"/env || die "Failed to create env file"
-		export PATH="\$HOME/.cabal/bin:${GHCUP_BIN}:\$PATH"
+		case ":\$PATH:" in
+			*:"${GHCUP_BIN}":*)
+				;;
+			*)
+				export PATH="${GHCUP_BIN}:\$PATH"
+				;;
+		esac
+		case ":\$PATH:" in
+			*:"\$HOME/.cabal/bin":*)
+				;;
+			*)
+				export PATH="\$HOME/.cabal/bin:\$PATH"
+				;;
+		esac
 		EOF
 
 	# shellcheck disable=SC1090
@@ -368,12 +381,38 @@ adjust_bashrc() {
 	case $1 in
 		1)
 			cat <<-EOF > "${GHCUP_DIR}"/env || die "Failed to create env file"
-				export PATH="\$HOME/.cabal/bin:${GHCUP_BIN}:\$PATH"
+				case ":\$PATH:" in
+					*:"${GHCUP_BIN}":*)
+						;;
+					*)
+						export PATH="${GHCUP_BIN}:\$PATH"
+						;;
+				esac
+				case ":\$PATH:" in
+					*:"\$HOME/.cabal/bin":*)
+						;;
+					*)
+						export PATH="\$HOME/.cabal/bin:\$PATH"
+						;;
+				esac
 				EOF
 			;;
 		2)
 			cat <<-EOF > "${GHCUP_DIR}"/env || die "Failed to create env file"
-				export PATH="\$PATH:\$HOME/.cabal/bin:${GHCUP_BIN}"
+				case ":\$PATH:" in
+					*:"\$HOME/.cabal/bin":*)
+						;;
+					*)
+						export PATH="\$PATH:\$HOME/.cabal/bin"
+						;;
+				esac
+				case ":\$PATH:" in
+					*:"${GHCUP_BIN}":*)
+						;;
+					*)
+						export PATH="\$PATH:${GHCUP_BIN}"
+						;;
+				esac
 				EOF
 			;;
 		*) ;;